4.4 Variations for Array Example

The code to call avg_of_double has two declarations that start with the same data type:

  /* The array of values to average.  */
  double nums_to_average[5];
  /* The average, once we compute it.  */
  double average;

In C, you can combine the two, like this:

  double nums_to_average[5], average;

This declares nums_to_average so each of its elements is a double, and average so that it simply is a double.

However, while you can combine them, that doesn’t mean you should. If it is useful to write comments about the variables, and usually it is, then it’s clearer to keep the declarations separate so you can put a comment on each one.

We set all of the elements of the array nums_to_average with assignments, but it is more convenient to use an initializer in the declaration:

{
  /* The array of values to average.  */
  double nums_to_average[]
    = { 58.7, 5.1, 7.7, 105.2, -3.14159 };

  /* The average, once we compute it.  */
  average = avg_of_double ((sizeof (nums_to_average)
                            / sizeof (nums_to_average[0])),
                           nums_to_average);

  /* …now make use of average… */
}

The array initializer is a comma-separated list of values, delimited by braces. See Initializers.

Note that the declaration does not specify a size for nums_to_average, so the size is determined from the initializer. There are five values in the initializer, so nums_to_average gets length 5. If we add another element to the initializer, nums_to_average will have six elements.

Because the code computes the number of elements from the size of the array, using sizeof, the program will operate on all the elements in the initializer, regardless of how many those are.